You're not wrong to say that. The shopfronts are usually well, shopfronts. A place for the customer service aspect to select fabric and take measurements. The actual tailoring is done by another offsite "factory"/"sweat shop". Heard some say that there're just a few mass production sewing facilities that nearly every tailor shop uses. Some others have their own little "factory".
The difference is perhaps that each tailor shop will have a set of tailors dedicated to them. While I believe that many of the "tailors" in the shopfronts are mere measurers and possibly can't thread a needle, some of the older generation of the established shops are likely to have started from ground up as a tailor and later ventured out.
It may also be that they now have become bosses they no longer do the more tedious sewing but have taken apprentice according to their craft for this. Some I heard continue to do the outlining and cutting. Once the shapes and markings are in place, it's likely that more experienced "sewers" would be able to bring out the product just by connecting the dots.
It's just like any aspiring fashion designer are expected to be able to do some sewing and such during their early days especially when under training. When they progress and move up, they just stop doing that. Do you think Valentino do the entire process himself after the achieved fame? He'd just do the design and make sure that the tailors in his employ deliver to his standard.
True that. At the end of the day..even the big brand names don't sew their own tailoring themselves..and I agree with condor..some of the older generations probably have little factories doing their clothes now with their own standards..so trust them to do their thing! Stick to the recommended stores and you should be fine! Don't just walk into any tout store or you'll just be doing another "random" pick thing and you never know how your clothes may turn out!!